The present invention relates to an extruder for highly foamed plastic material for coating a central conductor of a coaxial insulated cable or the like with an insulator of for example a highly foamed plastic material, preferably polyethylene, having at least a 70% or more foaming rate.
Conventionally, as the method for producing a coaxial insulated cable of the kind described above, there is a known technique in which hydrocarbon chloride fluoride (flon) or the like is used as a foaming agent in two extruders connected to each other. The foaming agent is led into the first-stage extruder at its middle portion so as to be kneaded with a plastic material such as polyethylene or the like, and then the kneaded plastic material is sent to the second-stage extruder so as to be further kneaded and the highly foamed plastic material is applied onto a central conductor as a coating.
FIG. 4 (Prior Art) is an external view showing an example of a screw used in the conventional first-stage extruder.
Here, the screw having a total length L is constituted by a feed portion 2, a kneading portion 3, a metering portion 4, a vent portion 5, a kneading portion 3', and a metering portion 4' having lengths 1.sub.1, 1.sub.2, 1.sub.3, 1.sub.4, 1.sub.2 ', and 1.sub.3 ' respectively.
A foaming-agent lead-in portion 6 is provided in a cylinder at a portion where the vent portion 5 of the screw 1 is positioned. The foaming-agent lead-in portion 6 has a structure such as a poppet valve, which is not illustrated in detail in the drawing, for preventing a plastic material in the cylinder from flowing back.
The pellet-like plastic material supplied from a portion (a) of the cylinder is kneaded through the lengths 1.sub.1, 1.sub.2, and 1.sub.3, further kneaded through the lengths 1.sub.2 ', and 1.sub.3 ' together with a foaming agent such as hydrocarbon chloride fluoride (flon) poured under volume-regulation at the vent portion 5, and then led into a second-stage extruder 10 (FIG. 3).
As portrayed in a schematic view in FIG. 3, the foaming agent and the mixed plastic material which are led into the second-stage extruder 10 are further kneaded so that the foaming agent is equally dispersed in the plastic material, and the thus kneaded highly foamed plastic material is applied onto a central conductor 16 as a coating therefor by a crosshead 15 constituted by a nipple and a die positioned at a top end of the second-stage extruder 10, thereby producing a highly foamed plastic insulated wire-like body 17.
Conventionally, in extruding a plastic material such as foamed polyethylene or the like in which hydrocarbon chloride fluoride (flon) or the like is used as a foaming agent, the foaming agent performs an expanding action onto the plastic material, so that the conventional screw performs sufficient kneading.
Recently, however, the theory has been advocated that hydrocarbon chloride fluoride, or flon, destroys the ozone layer, causing destruction of the ozone layer on a world-wide scale. Thus, in achieving a method for extruding a highly foamed plastic material using an inert gas (for example, a nitrogen gas) as a foaming agent, a conventional method for extruding a highly foamed plastic material cannot be used because a stable consistent foaming state is not achieved. When such a method is used for extruding a highly foamed plastic material having an inert gas, both the electrostatic capacitance required as a characteristic of cables and the outer diameter of the cables vary extremely. Accordingly, an extrusion method for extruding highly foamed plastic material using an inert gas as a foaming agent and in which the foaming state is consistent and stable has been required.